If you date someone that doesn't normally wear a ring , how did you figure out her ring size before you proposed? For the sake of argument, lets assume you know she would appreciate an engagement ring.
Most Helpful Girl
You could be sneaky... Is she a heavy sleeper? If so maybe wrap a piece of string or something around her finger and get the length. Measure it, and go to the store and ask what size it could be.
If you know she'd appreciate a ring, but still want it to be a surprise, and have no way of getting her ring size, maybe propose to her in a romantic way but explain you don't have the ring yet because you don't know her size. Or you could guess her size and simply get it refitted.
You could also ask her. It might ruin it, but if you ask her casually and then propose months later she could forget the entire thing by the time you do it.0